Internal combustion engines produce exhaust gases containing a variety of pollutants, including hydrocarbons, particulate matter, sulfur oxides, carbon monoxide, and nitrogen oxides (“NOx”), which include nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). Increasingly stringent national and regional legislation has lowered the amount of pollutants that can be emitted from such diesel or gasoline engines. Many different techniques have been applied to exhaust systems to clean the exhaust gas before it passes to atmosphere.
Exhaust gas recirculation (EGR) is a method for reducing NO emissions from an engine by returning a portion of the engine's exhaust gas to the engine combustion chambers via the air intake. EGR works by lowering the oxygen concentration in the combustion chamber, thereby decreasing the peak temperature of the fuel combustion flame as well as through heat absorption. EGR has been used since the mid-1970s in gasoline fueled passenger car engines. Following the gasoline application, EGR was also introduced to diesel passenger cars and, from the early 2000s, to heavy-duty diesel engines.
Generally, there are two exhaust system arrangements comprising EGR: (i) high pressure loop EGR, in which the exhaust gas is recirculated from upstream of a turbocharger; and (ii) low pressure loop EGR (also called long loop EGR), where exhaust gas is often recirculated from downstream of a particulate filter, allowing all the exhaust gas to be utilized in the turbo. Exhaust gas pressure downstream of the filter is generally lower than at the intake manifold, allowing exhaust gas to flow from the filter to the intake manifold.
The use of EGR systems has been taught in, for example, PCT Intl. Appl. WO 2012/120347 which discloses an exhaust system for a vehicular lean burn internal combustion engine that comprises a low pressure EGR circuit for connecting the exhaust system downstream of a filter to an air intake of the engine, wherein the EGR circuit comprises an ammonia oxidation catalyst. In addition, PCT Intl. Appl. WO 2012/114187 teaches an exhaust system for a vehicular lean burn internal combustion engine comprising a low pressure EGR circuit having a NOx adsorber catalyst (NAC) comprising a nitric oxide (NO) adsorbent.
As with any automotive system and process, it is desirable to attain still further improvements in exhaust gas treatment systems. We have discovered a new system that utilizes a N2O-producing catalyst within the EGR circuit in order to increase oxygen atom availability within the combustion process to reduce the number of unwanted diesel particulates.